1. Field of Technology
The disclosure relates generally to earth-boring bits used to drill a borehole for the recovery of oil, gas or minerals. More particularly, this disclosure relates to rolling cone drill bits having enhanced hydraulics and erosion-resistant cutting teeth.
2. Background Information
A conventional earth-boring drill bit is mounted on the lower end of a drill string. The bit is turned by rotating the drill string at the surface, by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and drills a borehole toward a target zone. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
One type of conventional bit includes one or more rolling cone cutters. As the bit is rotated, the cutters roll and slide upon the bottom of the borehole, breaking up the formation material. Typically, the cutting action of the cone cutters is enhanced by providing cutting elements (e.g., teeth) on the rolling cones. The borehole is formed as the action of the rolling cones and their cutting elements gouge, crush and shear formation material in the bit's path.
Rolling cone bits are typically characterized by the type of cutting elements employed on the rolling cones. A first type employs inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized holes formed in the cone surface. Such bits are typically referred to as “TCI” bits or “insert” bits. A second general bit type includes teeth that are milled, cast, or otherwise integrally formed from the material of the rolling cone, such bits being generally known as “steel tooth bits.”
While drilling, it is conventional practice to pump drilling fluid (also referred to as “drilling mud”) down the length of the tubular drill string where it is jetted from the face of the drill bit through nozzles. The hydraulic energy thus supplied flushes the drilled cuttings away from the cutters and the borehole bottom, and carries them to the surface through the annulus that exists between the tubular drill string and the borehole wall.
The cost of drilling a borehole is very high, and is proportional to the time it takes to drill to the targeted depth and location. In turn, the time required to drill the well is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation, as is necessary, for example, when the bit becomes worn or encounters formations for which it is not well suited to drill. The length of time before a drill bit must be changed depends upon its rate of penetration (“ROP”) as well as its durability. Whenever a bit must be changed, the entire drill string, which may be miles long and is made up of discrete sections of drill pipe that have been threaded together, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered back to the bottom of the borehole. This is accomplished by reconstructing the drill string, section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is desirable to employ drill bits that drill faster and longer, and that drill with an acceptable ROP over a wide range of formation types.
A drill bit's ROP and durability may be substantially affected by the design, placement and orientation of the nozzles in the bit face. For example, when drilling softer formations and plastic formations, cuttings tend to adhere to the cone cutters and between the cones' cutting elements, a phenomenon commonly referred to as “bit balling.” When bit balling occurs, the penetration of the individual cutting elements into the formation is restricted. With less penetration, the amount of formation material gouged or otherwise removed by the cutting elements is reduced, leading to a reduction in the bit's ROP. Also, formation packed against the cone cutters may close or greatly restrict the flow channels needed for the drilling fluid to carry away cuttings. This may promote premature bit wear. In either instance, having sufficient fluid flow can help to clean the cutting teeth, allowing them to penetrate to a greater depth, and to maintain the desired ROP.
A conventional nozzle arrangement includes the placement of a nozzle between each of the cone cutters and near to the cones' outermost row of cutter elements. Typically, the bit's hydraulics are designed such that each of these nozzles has the same orientation as the others that are similarly positioned. In other conventional designs, additional nozzles are positioned elsewhere in the bit body to direct a high velocity stream at other predetermined locations. However, conventional arrangements may not direct the hydraulic flow to the locations where cleaning is most needed and, for example, may not provide sufficient cleaning along the inner rows of the cones' cutting elements.
Further, drilling fluid, as it picks up and mixes with the drilled cuttings, becomes highly abrasive. The impact of the cutting-laden fluid directly on cutting teeth may severely erode the teeth. As with poor bit hydraulics, tooth erosion and/or loss of teeth may lead to a reduction in ROP and bit life, and necessitate a costly and premature trip of the drill string.
Accordingly, there is a need for bits having improved bit hydraulics that provide cleaning of cutting elements along the outer and inner rows of the cones in order to minimize bit balling and maintain acceptable ROP, without causing detrimental erosion of the cutting teeth.